The high-sensitivity detection of trace substances in a gas is becoming extremely important in diverse fields such as those of pollution prevention, specimen analysis, environmental monitoring and earth science. As one of conventional high-sensitivity detection apparatuses for trace substances in a gas, there is an apparatus utilizing a gas correlation method which has widely been used such as for measuring CO concentrations in exhaust gases of incinerators. The gas correlation method is a method of detecting gaseous trace substances which is prescribed by the United States Environmental Protection Agency (U.S. EPA), and is a sort of non-dispersive infrared absorption system. This method, which allows detection with high sensitivity as its effect of interference by a gaseous substance other than trace substances to be measured is limited and which is low in its apparatus cost, has widely been used in general.
Mention is now made of the conventional apparatus and concentration measuring method, which utilize the gas correlation method.
FIG. 8 is a diagrammatic cross-sectional view illustrating the makeup of a conventional apparatus for concentration measurement according to the gas correlation method (see Nonpatent Reference 1.). The apparatus 50 for concentration measurement according to the gas correlation method comprises: an infrared light source 51 of thermal radiation type; an optical system (collimator) 52 for collimating infrared light 51a generated by the infrared light source 51; a gas correlation filter 53 through which collimated infrared light 51a passes; a bandpass filter 54 for limiting a passband of the infrared light 51a passing through the gas correlation filter 53; a multi-reflection sample gas cell 55 in which a gas to be measured 55a is introduced or charged and through which infrared light 51a that has passed through the band-pass filter 54 passes; and an infrared detector 56 for measuring an intensity of infrared light 51a passing through the multi-reflection sample gas cell 55.
The gas correlation filter 53 consists of a gas cell 53a filled with an analyte gas at high concentration and a gas cell 53b filled with a gas not absorbing the infrared light, e. g., N2 gas. The gas cell 53a is used to form reference light excluding absorption spectral components of an analyte gas from infrared light 51a while the gas cell 53b is used to form probe light similar in level of light dispersion such as of Rayleigh scattering to the reference light. These gas cells are rotated about the central axis 53c of the gas correlation filter 53 to make such infrared light 51a successively incident on these two gas cells.
By selecting a passband of the bandpass filter 54 to be wider than and close as much as possible to an infrared absorption band of an analyte gas, it is possible to decrease an interference effect by a gas other than the analyte gas and to measure its concentration at high sensitivity. Here, the interference effect is meant to refer to an adverse effect on a measured value of the concentration of a analyte gas by that of a gas other than the analyte gas in the presence of skirt portions of the absorption spectrum of that other gas on those of the passband of the bandpass filter so as to cause infrared light of the passband of the bandpass filter to be absorbed by that other gas.
FIG. 9 carries charts illustrating principles of the conventional concentration measurements according to the gas correlation method. FIG. 9 (a) shows a spectrum formed by infrared light 51a passing through the bandpass filter 54, namely that of incident reference light that is incident on the multi-reflection sample gas cell 55. Numeral 61 designates a spectral defect caused by the absorption by an analyte gas filled in the gas cell 53a at high concentration while numeral 61a designates a spectral shape made up with the bandpass filter 54.
FIG. 9(b) shows a spectrum formed by infrared light 51a passing through the bandpass filter 54, namely that of incident probe light that is incident on the multi-reflection sample gas cell 55. Since the gas filled in the gas cell 53b absorbs no infrared light, it is shown that there is no such spectral defect.
FIG. 9(c) shows the spectrum of reference light detected by the infrared detector 56, which is shown damped by a loss in the optical system due to contaminations of mirrors in the multi-reflection sample gas cell 55 and their deviations of optical axes, namely by that other than an absorption loss by an analyte gas in the gas to be measured 55a. 
FIG. 9(d) shows the spectrum of probe light detected by the infrared detector 56, which is shown damped not only by a loss other than an absorption loss of an analyte but also by such an absorption loss of the analyte in the gas to be measured 55a. Numeral 62 indicates a damping by absorption of the analyte gas. The frequency domain in which the absorption occurs corresponds to that in which the spectral defect 61 in FIG. 9(a) occurs.
Since the loss in the optical system due to contaminations of the collimator 52, gas correlation filter 53 and bandpass filter 54 and their deviations of optical axes has no dependence on a frequency of infrared light and cause incident probe and reference light intensities Ip0 and Ir0 to be damped at an identical loss factor, ratio of the incident probe light intensity to the incident reference light intensity: Ip0/Ir0 is constant against their changes and also is constant against changes in output light intensity of the infrared light source 51. Here, since the incident probe light intensity Ip0 is proportional to an area of hatched portion in (b) and the incident reference light intensity Ir0 is proportional to an area of hatched portions in FIG. 9(a), Ip0/Ir0 represents a ratio in area of the hatched portion in FIG. 9(b) to the hatched portions in FIG. 9(a), that is a spectral area ratio.
Likewise, since the loss of the optical system based on contaminations of such as mirrors of the multi-reflection sample gas cell 55 and their deviations of optical axes in the optical system, namely the loss other than of absorption by an analyte gas damps incident reference and probe light intensities Ir0 and Ip0 at an identical loss factor, ratio: Ip/Ir, of probe light intensity Ip to reference light intensity Ir where they are detected by the infrared detector 56 is constant against their variations. Here, the probe light intensity Ip detected by the infrared detector 56 is proportional to an area of the hatched portion in FIG. 9(d) and the reference light intensity Ir is proportional to an area of the hatched portions in FIG. 9(c). Thus Ip/Ir is a ratio in area of the hatched portion in FIG. 9(d) to the hatched portions in FIG. 9(c), that is a spectral area ratio.
The reference light has not the absorption spectral component of an analyte gas and its intensity will in no case be damped by its absorption by the analyte gas in the gas to be measured 55a. Therefore, loss γ other than loss of absorption by the analyte gas in the multi-reflection sample gas cell 55 can be found from the ratio: Ir/Ir0, of the reference light intensity detected at the infrared detector 56 to the incident reference intensity as follows:
[Formula 1]γ=Ir/Ir0  (1)
The probe light intensity Ip detected at the infrared detector 56 has both the loss γ other than that of absorption by the analyte gas in the multi-reflection sample gas cell 55 and that loss of absorption by the analyte gas. Then, assuming that the degree of absorption by the analyte gas is α, the probe light intensity Ip can be expressed with using γ and the incident probe light intensity Ip0 by equation (2) below.
[Formula 2]Ip=γIp0e−α  (2)
Substituting γ in equation (2) with equation (1) gives equation (3) below.
[Formula 3]Ip=(Ir/Ir0)Ip0e−α  (3)
Equation (3) can be modified to give equation (4) below.
[Formula 4]Ip/Ir=(Ip0/Ir0)e−α  (4)
The equation (4) shows that the degree of absorption can be found from the ratio Ip/Ir of the probe and reference light intensities Ip and Ir detected by the infrared detector 56 and the ratio Ip0/Ir0 of the incident probe and reference light intensities Ip0 and Ir0 which can be measured when the apparatus is manufactured. Since as mentioned above Ip0/Ir0 is constant against changes in output light intensity of the infrared light source 51 and changes in loss in the optical system of the collimator 52, the gas correlation filter 53 and the bandpass filter 54 and Ip/Ir is constant against losses other than the loss of absorption by the analyte gas in the multi-reflection sample gas cell 55, a trace substance in a gas can be detected from the degree of absorption α found by this method, without being affected by such changes.
Nonpatent Reference 1: http://www.thermo.co.jp/tameninaru1-6. html
Nonpatent Reference 2: J. Faist, C. Gmachl, F. Capasso, C. Sirtori, D. L. Sivco, N. J. Baillargeon and A. Y. Cho: Appl. Phys. Lett. 70, 2670-2672 (1997)
Nonpatent Reference 3: C. Dmoto, N. Ohtani, K. Kuroyanagi, P. O. Baccaro, H. Takeuchi, M. Nakayama and T. Nishimura, “Intersubband Electroluminescence using X-Γ Carrier Injection in a GaAs/AlAs Superlatice”; Appl. Phys. Lett. 77, 848 (2000)
Nonpatent Reference 4: Y. Nishijima: J. Appl. Phys. 65, pp. 935-940
Nonpatent Reference 5: J. I. Malin, J. R. Meyer, C. L. Felix, J. R. Lindle, L. Goldberg, C. A. Hoffman, F. J. Bartoli, C.-H. Lin, P. C. Chang, S. J. Murry, R. Q. Yang, and S.-S. Pei: SPIE Vol. 2682, pp. 257-261 (1996)